Product User Guide (PUG) for the IMAP-DOAS XCH4 ... - GHG-CCI

ESA Climate Change Initiative (CCI)Product User Guide(PUG)Page 9Version113 Dec. 20123 Product Description3.1 Product Content and Formatfor the Essential Climate Variable (ECV)Greenhouse Gases (GHG)The IMAP XCH 4 data products are stored per orbit in a single NetCDF4 file. Retrieval resultsare provided for the individual SCIAMACHY spatial footprints, i.e. no averaging has beenapplied. A table with the full content of the data product is provided in Appendix A. Theproduct file contains the key products, i.e. the retrieved column averaged dry air mixing ratioXCH 4 . Information relevant for the use of the data is included in the data file, like the verticallayering and averaging kernels. Also, the parameters that are retrieved simultaneously withXCH 4 are included (e.g. retrieved CO 2 and CH 4 vertical column densities, fit quality, errorestimates3.2 Quality Flags and MetadataThere is a quality flag "/HighLevelResults/QualityFlag " included in the data file. However,recent analysis has shown that this flag may not be entirely adequate and a posteriorichanges to the quality flag were not feasible. Hence, we advise using a different filteringscheme based on several L2 data fields:• Land Fraction == 100 (/OrbitGeometry/land_fraction)• Reduced Chi 2 of CH4 fit < 10(/RetrievalResults/ch4_v60_esa_1pix/SpectralParameters/residual_reduced_chi2)• RMS of CH 4 fit < 0.005 AND >0(/RetrievalResults/ch4_v60_esa_1pix/SpectralParameters/residual_rms)• RMS of CO 2 fit < 0.004 AND >0(/RetrievalResults/co2_v60_v1/SpectralParameters/residual_rms)• Ratio of retrieved and prior CO 2 VCD >0.9 (simple cloud filter):VCD_CO2/VCD_CO2_prior>0.9with VCD_CO2: /HighLevelResults/ch4_v60_esa_1pix/co2_vcdVCD_CO2_prior: /HighLevelResults/ch4_v60_esa_1pix/co2_vcd_prior• Orbit quality of dark current correction:Max error < 3(/HighLevelResults/ch4_v60_esa_1pix/MaxErrorDarkCurrent)• Fit quality in Q-branch:Residual < 0.5(/HighLevelResults/ch4_v60_esa_1pix/MaxQBranchResidual_weighted)

ESA Climate Change Initiative (CCI)Product User Guide(PUG)Page 10Version113 Dec. 20123.3 Bias Correctionfor the Essential Climate Variable (ECV)Greenhouse Gases (GHG)As discussed previously, there may be a remaining bias depending on the amount ofatmospheric water vapour but this effect is not corrected in the dataset itself.3.4 Data UsageIt is strongly recommended to only use the custom quality filter described above.If the data are to be compared with other XCO2 and/or XCH4 data for which vertical profileinformation is available (e.g. inverse modeling, comparison to models, comparison tomeasured profiles), the column averaging kernels should be used. Here it should be notedthat the column averaging kernels are to be applied to layer sub-columns (molec/cm 2 ), asthese are the quantities directly retrieved in the IMAP-DOAS algorithm. For modelcomparisons the retrieved XCH 4 should be compared to [VCH4]' model /[VAIR] model where[VAIR] model is the total dry air column provided by the model and [VCH4]' model is the modeltotal CH 4 column after applying the column averaging kernel, viz.:'[VCH 4] model= [VCH 4] prior+ a T (x model− x prior)where [VCH4] prior is the prior CH 4 total column used in the retrieval, x model is the vertical CH 4profile from the model (as sub-columns in molec/cm 2 ) and x prior is the prior vertical profilefrom the retrieval. For application of the column averaging kernel the model vertical profileshould be re-calculated on the vertical grid of the retrieval (preferred) or the averaging kernelhas to be interpolated to the vertical grid of the model.3.5 Tools for Reading the DataThe data are stored in Netcdf4 format, which can be read with standard tools in the commonprogramming languages (IDL, Matlab, Python, Fortran90, C++, etc). Standard readers forHDF5 can also be used as netCDF4 is based on HDF5.Example code snippet (for python, more examples in python and C++ can be provided onrequest):import h5pyf = h5py.File(file, "r")# get geolocationslat = f['OrbitGeometry/center_latitude'][:]lon = f['OrbitGeometry/center_longitude'][:]# XCH4:xch4 = f['HighLevelResults/ch4_v60_esa_1pix/XCH4_retrieved'][:]f.close()

ESA Climate Change Initiative (CCI)Product User Guide(PUG)Page 11Version113 Dec. 20123.6 Known Limitations and Issuesfor the Essential Climate Variable (ECV)Greenhouse Gases (GHG)• General retrieval quality deteriorates after November 2005 due to higher noise inmost important detector pixel (see Frankenberg et al, 2011)• After around mid 2010, there seems to be a sudden jump in XCH4 values in v60 (seeFigure 2.2). The origin is still unclear but most likely related to a detector pixelbehavior step-change (which may have been flagged bad in v55).• The new version based on only one detector pixel in the Q-branch (instead of 3) mayhave different susceptibility to water vapor interferences (see above and Houwelinget al 2013, in preparation).3.7 Data file contentTable 3-1: Data content (reduced to most important parameters) of the CH4 IMAP-DOASproduct. Dimensions are n: number of retrievals per orbit, m: number of fitting layersName Type Dimensions Units Description/OrbitGeometrycenter_latitude Float n degrees Geodetic latitude of pixelcentrecenter_longitude Float n degrees Geodetic longitude of pixelcentreline_of_sight_zenith Float n degrees Line of sight zenith anglesolar_zenith Float n degress Solar zenith anglesolar_azimuth Float n degress Solar Azimuth anglesatellite_azimuth float n Degrees viewing azimuth angleintegration_time Float n seconds Integration timecorner_latitudes Float (n,4) degrees Latitude of the 4 corners of theground-pixelcorner_longitudes Float (n,4) degrees Longitude of the 4 corners ofthe ground-pixelsurface_altitude Float n meter Ground pixel surface elevationAirMassFactor Float n - Air Mass Factor1/cos(SZA)+1/cos(LZA)orbitalPhase Float n orbital phase (from L1B file)scan_range Float n Degree Absolute range of line of sightzenith anglespet Float n seconds Pixel Exposure Timedry_air_column_ecmwf Float n Molec/cm 2 Dry air column derived fromECMWFh2o_air_column_emcwf Float n Molec/cm 2 H 2O air column derived fromECMWFtime_mjd Double n Days sinceModified Julian Day1.1.2000 00:00surface_pressure Float n hPa Surface Pressure (fromECMWF)/HighLevelResults/ch4_v60_esa_1pixch4_vcd Float n molec/cm 2 Methane vertical columndensitych4_vcd_sigma Float n molec/cm 2 1-sigma precision error inmethane vertical columndensityco2_vcd Float n molec/cm 2 Carbon dioxide vertical

ESA Climate Change Initiative (CCI)Product User Guide(PUG)Page 12Version113 Dec. 2012for the Essential Climate Variable (ECV)Greenhouse Gases (GHG)column densityco2_vcd_sigma Float n molec/cm 2 1-sigma precision error inCO 2 vertical columndensityxCH4_retrieved Float n ppb Derived column averageddry methane mixing ratioxCH4_1sigma Float n ppb 1-sigma precision error inxVMR_CH4xCH4_prior Float N ppb Prior XCH 4 from TM4co2_column_averaging_kernelch4_column_averaging_kernelFloat n x m - CO 2 total columnaveraging kernel for 10layers per soundingFloat n x m - CH 4 total column averagingkernel for 10 layers persoundinglayer_ch4_prior_vcd Float n x m Molec/cm 2 CO 2 prior VCD profile (10layers)layer_co2_prior_vcd Float n x m Molec/cm 2 CH 4 prior VCD profile (10layers)layer_dry_air_vcd Float n x m Molec/cm 2 Dry air VCD per layer (10layers, to derive VMRsfrom prior trace gas VCDs)layer_lower_level Float n x m hPa Lower pressure levels of mretrieval layerslayer_upper_level Float n x m hPa Upper pressure levels of mretrieval layersQualityFlag Float n - Quality flag or retrieval -1=failed, 0=converged butprobably cloudy,1=converged andsupposedly cloud freeMaxErrorDarkCurrent Float n BU Maximum absolute readoutof Q-branch detector pixelat SZAs>95 degreesMaxQBranchResidual Float n radiance Maximum absoluteresiduum in the CH 4 Q-branch pixelMaxQBranchResidual_weightedch4_v60_esa_1pix/SpectralParameters/residual_rmsch4_v60_esa_1pix/SpectralParameters/residual_reduced_chi2co2_v60_v1/SpectralParameters/residual_rmsco2_v60_v1/SpectralParameters/residual_reduced_chi2ch4_v60_esa_1pix/Ancillary/iterationsco2_v60_v1/Ancillary/iterationsFloat n radiance Maximum weighted (bynoise) residuum in the CH 4Q-branch pixel/RetrievalResultsFloat n - fit residual RMS for theCH 4 fit windowFloat n - fit reduced chi 2 for the CH 4fit windowFloat n - fit residual RMS for theCO 2 fit windowFloat n - fit reduced chi 2 for the CO 2fit windowFloat n - number of iterations usedin the CH 4 fitFloat n - number of iterations usedin the CO 2 fit

ESA Climate Change Initiative (CCI)Product User Guide(PUG)Page 13Version113 Dec. 2012for the Essential Climate Variable (ECV)Greenhouse Gases (GHG)4 References and Further ReadingBergamaschi, P., (null), Meirink, J. F., Krol, M., Villani, M. G., Houweling, S., et al. (2009).Inverse modeling of global and regional CH4 emissions using SCIAMACHY satelliteretrievals. Journal of Geophysical Research-Atmospheres, 114, D22301.doi:10.1029/2009JD012287Bergamaschi, P., Frankenberg, C., Meirink, J. F., Krol, M., Dentener, F., Wagner, T., et al.(2007). Satellite chartography of atmospheric methane from SCIAMACHYon boardENVISAT: 2. Evaluation based on inverse model simulations. Journal of GeophysicalResearch-Atmospheres, 112(D2), D02304. doi:10.1029/2006JD007268Bloom, A. A., Palmer, P. I., Fraser, A., Reay, D. S., & (null). (2010). Large-Scale Controls ofMethanogenesis Inferred from Methane and Gravity Spaceborne Data. Science, 327(5963),322–325. doi:10.1126/science.1175176Butz, A., Hasekamp, O. P., Frankenberg, C., Vidot, J., & Aben, I. (2010). CH4 retrievals fromspace-based solar backscatter measurements: Performance evaluation against simulatedaerosol and cirrus loaded scenes. Journal of Geophysical Research-Atmospheres, 115,D24302. doi:10.1029/2010JD014514Frankenberg, C., Aben, I., Bergamaschi, P., Dlugokencky, E. J., van Hees, R., Houweling,S., et al. (2011). Global column-averaged methane mixing ratios from 2003 to 2009 asderived from SCIAMACHY: Trends and variability. Journal of Geophysical Research-Atmospheres, 116, D04302. doi:10.1029/2010JD014849Frankenberg, C., Meirink, J., Bergamaschi, P., Goede, A., Heimann, M., Korner, S., et al.(2006). Satellite chartography of atmospheric methane from SCIAMACHY on boardENVISAT: Analysis of the years 2003 and 2004. Journal of Geophysical Research-Atmospheres, 111(D7), D07303. doi:10.1029/2005JD006235Frankenberg, C., Bergamaschi, P., Butz, A., Houweling, S., Meirink, J. F., Notholt, J., et al.(2008). Tropical methane emissions: A revised view from SCIAMACHY onboard ENVISAT.Geophysical Research Letters, 35(15), L15811. doi:10.1029/2008GL034300Frankenberg, C., Meirink, J., van Weele, M., U Platt, & Wagner, T. (2005a). Assessingmethane emissions from global space-borne observations. Science, 308(5724), 1010–1014.doi:10.1126/science.1106644Frankenberg, C., U Platt, & Wagner, T. (2005b). Iterative maximum a posteriori (IMAP)-DOAS for retrieval of strongly absorbing trace gases: Model studies for CH4 and CO2

ESA Climate Change Initiative (CCI)Product User Guide(PUG)Page 14Version113 Dec. 2012for the Essential Climate Variable (ECV)Greenhouse Gases (GHG)retrieval from near infrared spectra of SCIAMACHY onboard ENVISAT. AtmosphericChemistry and Physics, 5, 9–22.Frankenberg, C., Warneke, T., Butz, A., Aben, I., Hase, F., Spietz, P., & Brown, L. R. (2008).Pressure broadening in the 2 nu(3) band of methane and its implication on atmosphericretrievals. Atmospheric Chemistry and Physics, 8(17), 5061–5075.Schepers, D., Guerlet, S., Butz, A., Landgraf, J., Frankenberg, C., Hasekamp, O. P., et al.(2012). Methane retrievals from Greenhouse Gases Observing Satellite (GOSAT) shortwaveinfrared measurements: Performance comparison of proxy and physics retrieval algorithms.Journal of Geophysical Research, 117(D10), D10307. doi:10.1029/2012JD017549Tran, H., Hartmann, J. M., Toon, G., Brown, L. R., Frankenberg, C., Warneke, T., et al.(2010). The 2 nu(3) band of CH4 revisited with line mixing: Consequences for spectroscopyand atmospheric retrievals at 1.67 mu m. Journal of Quantitative Spectroscopy andRadiative Transfer, 111(10), 1344–1356. doi:10.1016/j.jqsrt.2010.02.015